This invention relates to self-twist yarn and an improved process and apparatus for forming it.
In the manufacture of yarn, particularly from synthetic fibers, there have been substantial developments in the areas of false-twist and self-twist yarns because of various production advantages which can be realized using these techniques. Such processes provide a shortened manufacturing route to a finished yarn product, and are therefore more economical as compared with conventional spinning and twisting processes.
As used herein, the term "false-twist" refers to a yarn in which a yarn strand is twisted by a twist insertion device to generate opposite twists on either side of the device. The point in the strand where the twist reverses has zero twist and will be referred to as a "node." The directions of twist are referred to as "S-twist" or "Z-twist," the appropriate letter being employed for twists in which the helices are twisted strands correspond with the middle portion of the appropriate letter.
The term "self-twist" is applied to yarns wherein two or more false twisted strands are brought together and permitted to ply themselves. The approximately equal torsional force of the same direction is stored in two or more singles yarns which are later brought into contact. Torque is released, permitting the single yarns to untwist, and in so doing, wrap around each other to form a plied yarn.
Generally speaking, false-twisting and self-twisting and the yarns produced thereby have received considerable attention in recent years. Our U.S. Pat. Nos. 4,074,511, 4,104,855 and 4,123,893 relate to this subject, and disclose but do not claim the invention claimed herein. Reference is also made to the following documents which describe these yarns, the techniques for producing them, and specific apparatus related thereto:
"Self-Twist Yarn", D. E. Henshaw, Merrow Publishing Co. Ltd., Watford, Herts, England, 1971.
______________________________________ RE 27,717 Breen et al 3,225,533 Henshaw 3,306,023 Henshaw et al 3,353,344 Clendening, Jr. 3,434,275 Backer et al 3,488,939 Walls 3,507,108 Yoshimura et al 3,537,251 Kimura et al 3,717,988 Walls 3,775,955 Shah 3,940,917 Strachan 4,055,039 Movshovich et al 4,068,459 Movshovich et al 4,084,400 Movshovich et al. ______________________________________
While this is by no means an exhaustive listing of patents or literature references on this subject, these references provide a foundation for the principles and techniques of this invention.
As will be recognized from these and other references relating to this art, there are a number of problems inherent in producing yarn using self-twist techniques, these problems being related in part to the fact that the yarn tends to be relatively unstable due to the different twists in singles being able to cancel each other through the node area. In this regard, the above-cited U.S. Pat. No. 3,434,275, to Backer et al suggests joining regions of twist reversal. Also, in the production of self-twist yarn, the yarn tension and other parameters involved in the production are highly critical and must be closely controlled.
When a continuously moving pair of single strands are twisted, brought together and allowed to ply immediately, the plied yarn has been found to exhibit non-uniform twist distribution. Generally, the twist is tighter just after the twist direction change at the node, and then begins to decrease with increased distance from the node. In some cases, a distinct loss of twist has been observed just prior to the direction change node.
The tight twist presence preceding the node can be attributed to feed-through of backed up twist from behind the insertion device when the twist direction change occurs. Because the ply twist is the result of the release of forces stored in the singles twist, the twist non-uniformity in the plied yarn is apparently caused by non-uniformity of the singles twist. This is partly the result of twisting the singles yarn while it moves through a reversible twist insertion device such as a reversible jet twister. The strand is twisted in one direction, generating, for example, a Z twist behind the jet and an S twist ahead of the jet, and then reversing the direction of the twist insertion device so that, at the instant of the switch from Z to S ply mode, the jet permits the leading end of the upstream Z singles twist to pass through to a position downstream of the jet. After reversal, the jet further inserts Z twist ahead of the jet in a portion of the yarn which already has some Z twist, thereby causing the portion adjacent the node to be more tightly twisted than the following portions. This is also true when the twist is in the opposite direction.
Clearly, such uneven twist creates differences in stored torque along the length of twist between the nodes. The yarn cross-sectional areas (fibers per cross-section) are equal or nearly equal. Since one portion is twisted tighter than other portions, it has greater stored torque and therefore a greater tendency to untwist than the other portions.
It will be observed that the yarn twist cannot be redistributed after plying because each cross-section in a self-twist yarn has reached a torque balance between the ply and singles twist. Once this balance occurs, no further axial rotation can occur.
This invention provides a solution to some of the production speed and quality control problems which have been experienced in the prior art. The invention provides for an uncomplicated machine and process operable at relatively high speeds to produce a quality yarn. The coincidence between production speed and yarn quality is attributable in large part to the fact that after the single strands are twisted, their twist is permitted to level or redistribute itself before the strands are set free to ply about each other.